Exhaust gas recirculation (EGR) systems are now commonly found in internal combustion engines. As it is well known, EGR systems can be utilized to control the cylinder charge and therefore the combustion process. The exhaust gas recirculated to the intake manifold (the amount of which can be regulated via an EGR valve) increases the proportion of inert gas in the fresh gas filling. This results in a reduction in the peak combustion temperature and, in turn, in a drop in temperature-dependent untreated NOx emissions.
It is desirable to be able to check the functioning of an EGR system. U.S. Pat. No. 5,632,257 relates to a method of diagnosing an EGR valve, wherein the EGR valve is forcibly operated in open/closed positions. An estimation of whether or not the actual exhaust gas recirculation quantity has changed with the forcible operation of the EGR valve is then made based on the corresponding variation in combustion pressure. This variation in combustion pressure is monitored as a change in IMEP.
In some engines, the EGR system comprises an EGR cooler that allows cooling the exhaust gas traveling to the intake manifold. The EGR cooler typically comprises a bypass valve that allows bypassing the EGR cooler (i.e. there is no flow of exhaust gas through the cooling part) so that, in effect, the bypass valve operates as an on/off valve for the cooler.
A difficulty that however arises with such EGR coolers is the impossibility of checking the proper functioning thereof. Indeed, contrary to the EGR valve, the bypass valve is normally not provided with a position sensor. Neither is there any temperature sensor at the EGR cooler outlet or in the intake manifold that would permit checking that the EGR gas has been cooled.
GB 2473602 describes a method for the diagnosis of the EGR cooler efficiency in a diesel engine, which employs a model for determining the temperature drop in the EGR cooler and that applies the so-called “Statistical Local Approach”. This model is able to correlate the efficiency of the cooler with the gas temperature and pressure values in the exhaust and intake manifold. Hence, temperatures at the EGR cooler inlet and outlet are required, as well as inlet and outlet pressures. This is thus a complex system to perform EGR cooler diagnostic that requires many input variables.